Abstract

A novel fluidized bed gasification reactor has been developed to get a product gas with a high calorific value (up to 15 MJ/Nm³) and nearly free of nitrogen. The gasification process is based on an internally circulating fluidized system and consists of a gasification zone fluidized with steam and a combustion zone fluidized with air. The circulating bed material acts as heat carrier from the combustion to the gasification zone. Gas mixing between these two zones is avoided by construction measures. Furtheron, the apparatus is characterized by a very compact design. This process was developed by the Institute of Chemical Engineering, Fuel and Environmental Technology in cooperation with AE Energietechnik (A patent for this system with the Number 405937 was granted).
The development of the gasification reactor has been carried out step by step. First, a cold flow model was operated to study the fluid mechanics of the fluidization system. The second step was a laboratory scale test rig to study the main features of the reactor by varying different operating and geometrical parameters. After this step a pilot plant was constructed and has been successfully operated. The results attained came fully up to the expectations.
Keywords: Biomass, steam gasification, fluidized bed, compact reactor design, medium energy gas, low nitrogen content.

Introduction

In Europe, Austria is one of the leading countries in using bioenergy. The most common utilization of biomass for energy is the combustion for heating applications. Gasification could become a second important route especially for power production.
Usually, biomass gasification is carried out using fixed or fluidized beds. As the overall gasification reactions are endothermic, the gasification process must be supplied with heat. The easiest way is to use air as gasification agent and to burn the biomass partially within the gasification reactor. In this case the product gas has a low calorific value (around 4-6 MJ/Nm³) and a high nitrogen content of 45-55 %.
A gas with a low nitrogen content and a higher calorific value (about 12 MJ/Nm³) can be produced with pure oxygen as gasification agent but the costs for the oxygen production are high. Another possibility is to supply heat with heat exchangers but here material problems due to the high temperature level will arise. The dilution of the product gas by nitrogen can also be avoided by using a dual fluidized bed system. In this case no oxygen generator is necessary and also no serious material problems due to high temperatures will appear..

Overview about the pilot plants

The Test Rig and Pilot Plant

The gasification process has been developed step by step. Five steps are carried out already and a demonstration step is in operation at the Biomass-CHP in Guessing:

Cold Flow Model

At the beginning of the development a cold flow model was built to study and optimize the fluid mechanics of an internally circulating fluidized bed with a draught tube and a surrounding annular bed. Circulation rates of the bed material and gas leackage between the two zones were measured. The circulation rates are important for the heat transport. Furtheron the gas leackage must be minimized. The results were very promising therefore a laboratory scale test rig for gasification tests was constructed.

Laboratory Test Rig

The laboratory test rig was designed for a thermal output of 10 kW. Fuel is fed into the annular bubbling bed by a double screw feeding system. The annular bed is fluidized with steam which also acts as gasification agent. The bed material together with the charcoal moves down towards a riser which is situated in the centre of the reactor. The particles are transported up by air through the riser where charcoal is partly burned. At the top of the riser the particles are separated from the gas. The particles fall down and come back to the gasification zone via an annular gap. This gap and the location of the steam and air inlets ensure, that the gas leackage between the two zones is lower than 5 % of the total gas input. Both zones have separated gas exits.

1st Pilot Plant

The next step was the construction of a pilot plant with a thermal output of about 100 kW based on the experience of the laboratory test rig. The circular-symmetric geometry was changed to a rectangular cross-section because of scale up consideration. The riser and the bubbling gasification bed were arranged side by side. To avoid large amounts of gas mixing a siphon was introduced in the line from the combustion zone to the gasification zone. The bed particles were splitted from the riser gas stream using a U-beam separator. The fuel feeding system consisted of a hopper and a multi-screw-conveyor. Air was supplied by blowers into the riser and during the start up period also into the gasification zone. Steam was produced by an electrical steam generator and overheated by an electrical heater. The product gas, which is cleaned by a cyclone and afterwards cooled from 700°C to 200°C by an heat exchanger, and the flue gas had separated exits from the reactor. They were mixed together after taking gas samples for analyzing. A gas burner ensured that the product gas was completely combusted before entering a cyclone and a flue gas cooler. The flyash could be returned continuously into the gasifier with the aid of a pneumatic flyash recycle system.
This pilot plant was used for experiments from May 1995 till August 1999.

2nd Pilot Plant

From the results of the 100kW pilot plant a second pilot plant with an improved operational performance and a thermal input of also 100kW was designed and constructed. This pilot plant was put into operation in November 1999.
This pilot plant was manily used to investigate different gas cleaning systems. With the results of the second pilot plant the design of the 8MW fuel input demonstration plant was done.

3rd Pilot Plant

The 3rd pilot plant is very similar to the second one. The main difference is that the separation at the bottom between gasification and combustion zone now is done by a siphon.

Overview about commercial gasifiers

Beside the biomass CHP Güssing, there are already several other biomass gasifiers based on FICFB gasification in operation. Most are using the syngas in gas engines, the GoBiGas project in Sweden is converting it to BioSNG (synthetic natural gas).

Location
Gas utilisation / Product
Fuel input / Product
MW, MW
Start up
Status
Supplier
Güssing, AT
Gas engine
8.0 / 2.0
2002
Operational
AE&E / Repotec
Oberwart, AT
Gas engine / ORC
8.5 / 2.8 
2008
Operational
Ortner Anlagenbau
Villach, AT
Gas engine
15 / 3.7 
2010
on hold
Ortner Anlagenbau
Senden/Ulm, DE
Gas engine / ORC
14 / 5 
2011
Operational
Repotec
Burgeis, IT
Gas engine
2 / 0.5
2012
Operational
Repotec
Göteborg, Sweden
BioSNG
32/20
2013
Operational
Repotec / Valmet
California
R&D
50/30
2015
Operational
GREG

Advantages of this gasification system

A novel gasifier was presented which has the following main advantages compared with an air blown gasifier:

Possible applications of the Product Gas

The product gas, which is produced, by this novel gasification system can be used in the following applications: